keyframe slide on timeline
Try fcurves.find...you need to access it from object.data.animation_data instead of object.animation_data (since bevel start end are part of data block). You can find the data path by right clicking on the property in the edit properties panel.
for o in bpy.context.selected_objects :
    action = o.data.animation_data.action # Note: o.data.animation_data
    fcurve = action.fcurves.find('bevel_factor_start'):
    for keyframe in fcurve.keyframe_points:
        keyframe.co[0]=keyframe.co[0]+10

```